Effects Of poor synchronization of two AC Generators or Alternators

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Poor synchronization creates damage to the equipment and power system network. So it needs to be done with perfect matching of frequency, phase and magnitude. The following statements explain the effects of improper matching of each parameter while synchronizing

Effects of poor frequency matching

In a generator, the rotor rotates with the help of prime mover (turbine- steam, hydraulic, gas etc) produces the rotating magnetic field which will cut by the stator winding and induces the EMF. Therefore the frequency of the induced EMF or voltage is proportional to the speed of the rotor.


i.e N= 120 F/P

Where N = Speed of the generator

           P = Poles of the generator

          F = frequency of the generator

So to control the frequency of the generated voltage, the speed can be varied.

Effects of poor synchronization

Effects of poor synchronization

Consider the generator has to be connected to power system network (Group of generators) i.e. the group of generators are running at same frequency to avoid the mismatch and one more generator has to be connected to this group. The rotating force of group of generators will be large compared to the single generator rotating force. So after the generator is connected to the power system network the speed or frequency of the generator is governed by the power systems network i.e. the power system network push the incoming generator in to its synchronized rotating condition.

So prior to closing the circuit breaker if the rotor speed or the frequency of induced voltage deviates from the power system network frequency i.e. if they are not closely matched, the power system with high rotational force pulls the generator in to it due to the improper frequency matching.  Due to this pulling, a sudden acceleration or deceleration effects will imposed on the rotor and thus the prime mover rotating toque. This transient in rotating torque of the prime mover increases the rotating mass forces on the shaft which may damages the shaft body and rotor over a life period.

In addition to this due to this high torque there may sudden increase in current flowing through the generator and generator transformer winding which may create serious damage to the winding. So the windings have to be designed to with stand these types of forces. There will be oscillations in the power generation and scope of operation of power system protection also. In a summarized way the poor frequency matching may result the following effects in the power system.

  1. High Acceleration or deceleration in the prime mover which increases the transient torque and creates damage to the rotor.
  2. High transient current may flow in the windings
  3. Power system oscillations due to the transient effect
  4. Possible power system protection operation which creates power system outage.

If only two generators have to be connected the generator with high power and rotational force tries to pull the other one if any improper matching occurs. This may create a serious damage to the both generators.

Effects of the improper phase matching in Syncronization

The phase and frequency are closely related in the power system i.e. the phase can be controlled by the rotor angle with the stator which can be controlled by the rotor rotation. Even if the frequencies of two power sources are matching together the phase angle should be matched to avoid the damage to the power source. If the two phase angles are not in match then there will be a difference in rotor angle and stator flux angle as shown in figure.

This difference causes a high transient torque when it synchronized to the grid power supply due to pull in force of large power system. Even more damage to the rotor mass and generator windings compared to frequency difference.

In addition to this when the unit is synchronized with improper phase angle match with the power system network, there will be high resultant transient voltage at an instant which may damage the insulation of the equipment. In summarized way poor phase matching during synchronization causes the following effects in power system.

  1. High transient toque with high current which damages the windings of the generator.
  2. High instant voltage which may damage the insulation of equipment
  3. Power system protection operation.

Effects of improper Voltage magnitude matching in Syncronization

Magnitude is the terminal is the one of the matching parameter during synchronization. The magnitude can be varied by varying the excitation of the generator. Low magnitude means low excitation high magnitude means high excitation. This magnitude difference two power system should be in safe limits for a perfect synchronization. If the incoming voltage is higher compared to running voltage then there will be high reactive flow from the generator which can damage the generator shaft and high mechanical forces on stator winding. If the incoming voltage magnitude is lower that implicates weak excitation field. Once the machine synchronized due to this less excitation there will high reverse current from stator to rotor which produces a double rotor current in the rotor windings. If this currents are high then there will serious damage to the windings. There is possibility of slip protection operation during this condition which may interrupt the power generation. In a summarized way effect of poor voltage magnitude matching during synchronization are as follows:

  1. Due to high incoming voltage magnitude, high reactive power will flow through the generator which gives high mechanical shock to the stator and rotor shaft.
  2. Due to low incoming voltage magnitude results in weak magnetic flux which may cause reverse current and slip protection of the generator.

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